The current approach to the construction of large highrise commercial or residential buildings is to first construct a self-supporting structure of a roof, floors and interior bearing members (including posts, beams, bearing walls, columns, and other structural supports), generally out of concrete and/or steel, and then to encase this structure with an exterior shell. The exterior shell provides an insulating, weather-proof, generally air-tight and aesthetic cladding, but essentially no structural strength. The two most common types of exterior shell systems for such buildings are called “window wall” systems and “curtain wall” systems.
In window wall systems, rigid panels of a manageable size and weight are prefabricated to roughly the same height as the spacing between adjacent pairs of concrete floor slabs. These rigid panels are inserted between the concrete floor slabs and are sealed with caulking. These rigid panels can take on a variety of exterior appearances but in general they consist largely of clear glass panels (i.e. windows, which are referred to as “vision glass” in the industry) along with opaque glass or metal infill panels (these opaque glass panels are generally referred to as “spandrel glass”). These rigid panels are framed with horizontal and vertical runs of metal mullions, headers, sills and trim as required. Most window wall systems are constructed of aluminum, although some may be of steel.
A vertical cross-section of a typical window wall system is presented in FIG. 1. In the interest of simplicity, the rigid panels 10 are shown to consist of vision glass only, but combinations of vision glass, spandrel glass, and other finishing materials are also often used. As shown, the rigid panels 10 are framed with top channels 12 and bottom channels 14, and vertical mullions (not shown). The framed, rigid panels 10 are then placed between adjacent pairs of concrete floor slabs 16, 16′. In such an installation, caulking 18, 18′ is required at both the top and bottom of the rigid panels 10. In this figure, the faces of the concrete slabs are finished with a metal panel 20, which will typically be painted.
Window wall systems suffer from a number of problems the most significant of which is poor long-term performance. As noted above, regular window wall are sealed with caulking between the rigid panels and the floor slabs—eventually this caulking is going to break down and leakage will occur. The concrete floor slabs keep moving, mainly through winter and summer expansion and contraction cycles. This cycling keeps compressing and decompressing the rigid frames repeatedly, placing a strain on the caulking until it begins to break down. The length of time that it takes for this breakdown to occur depends on the environment and the specifics of the installation, but these components often start opening up after two to five years, thus requiring maintenance. Once the caulking joints start opening up, water will get into the system and damage both the window wall system and the building interior. This deterioration and long-term performance is a severe problem with window wall systems.
Of course, the caulking must also be installed properly in the first place, which is not an easy task. Even pinholes or small cracks may allow water to leak into the building, particularly on the windward side of the building which experiences significant levels of air pressure across the cladding system. If too thin a layer of caulking is applied, or if the surfaces are dirty, oily or wet when the caulking is applied, the seal may fail very quickly or be ineffective right from the beginning.
Conventional curtain wall systems are similar to window wall systems in that they consist of prefabricated rigid panels which form a non-structural exterior cladding for the building. Like window wall systems, these rigid panels consist largely of vision glass with suitable infill panels and framework. They are sized to be of manageable size and weight, but are taller than window wall panels because they equal to the building's story height, rather than the distance between the slabs.
A vertical cross-section of a typical curtain wall system is presented in FIG. 2. In this case, each pre-fabricated rigid panel 30 has been shown to consist of both vision glass 32 and spandrel glass 34, but any combination of vision glass, spandrel glass, sheet metal panels and other finishing materials may also be used. The rigid panels 30 are framed with a bottom channel 36 and vertical mullions (not shown). Each bottom channel engages with the sprandrel glass 34 of the rigid panel 30 mounted below it.
The big difference between curtain wall systems and window wall systems is that the rigid panels 30 of the curtain wall system are hung on the building structure, usually from floor to floor, each module being supported by connectors on the outer area of each concrete floor slab 16, 16′. Rigid curtain wall panels are stacked on each other in parallel rows and adjacent modules are typically connected together.
That is, rather than the rigid panels fitting between the concrete slabs 16, 16′ as in the case of window wall systems, they hang from the slabs like curtains, sitting proud of the slab with each panel being sealed to the next with gaskets. In this arrangement, the rigid panels 30 of the curtain wall system are sealed to one another which minimizes the effect of thermal cycling. Hence, this makes a shell which is essentially continuous, and entirely outside the structure of the building.
Conventional curtain wall systems generally do not have the caulking/leakage problem of window wall systems, but they have problems of their own.
To begin with, conventional curtain wall systems have a gap between the vision glass 32/spandrel glass 34 layer and the faces of the concrete slabs 16, 16′ which introduces problems with regard to sound transmission, and smoke and fire safing between individual floors. While these issues can be addressed with suitable sealing systems, the larger the gaps are, the more expensive and time consuming they are to fill and they result in a possible failure point for the system.
A related problem is that conventional curtain wall systems have vertical mullions which run continuously from the bottom to the top of the building. These vertical mullions are hollow and boxed-shaped in cross-section, so they essentially act like a duct for the flow of sound and smoke between floors. The interiors of these vertical mullions are not sealed between floors and it would be very expensive and inconvenient to do so.
Also, conventional curtain wall systems require anchors 38 which are embedded directly in the concrete floor slabs to support their rigid panels 30. The supply, layout and installation of these embedded anchors 38 is a costly item, particularly in high labor cost markets. Curtain wall systems are mounted to the top of, or on the face of the concrete floor slabs 16, 16′ with one embedded anchor 38 taking care of each vertical mullion. Because each embedded anchor 38 is supporting a great deal of load, including both vertical loads and in/out loads, these embedded anchors are 38 typically cast into the concrete floor slabs 16, 16′. Expansion anchors and other types of concrete fasteners simply are not strong enough to support such loads reliably.
Attempts have been made to address the problems of conventional window wall and curtain wall systems. For example, window wall systems have been provided with an upper channel which runs along the underside of the upper floor slab, the rigid panels 10 being fitted into this channel. This allows for some vertical movement, but the interface of the rigid panel and the horizontal channel still has to be caulked, so it will eventually break down; and this additional interface introduces another point for possible air and water leakage. None of these modified window wall or curtain wall systems have been very successful.
Recently, high-end residential condominium developers, building envelope consultants and architects in Canada and the USA have been asking for a low cost curtain wall system for use in their projects to replace commonly used window wall systems. The uniform response from developers and architects was that they are tired of the poor long-term performance of window wall systems and of the standard “prison look” effect due to restrictions in module width and wide mullion assemblies. Also, the appearance of slab-edge metal cover panels 20 was not aesthetically pleasing.
Because window wall systems are installed between the concrete floor slabs, there is necessarily a horizontal panel 20 at least at every floor slab. As well, because window walls are being designed with very small and weak vertical mullions in the interest of cost reduction, they must be supplied with a larger number of vertical mullions, resulting in more vertical lines. Rather than using four- or five-foot spacings for vertical mullion modules, it is now common to see vertical mullions spaced on three-foot to four-foot centres. The large number of vertical mullions creates a prison-like look with these vertical lines every three feet or so.
Any new window wall or curtain wall system must also take installation cost into consideration. Installers are expensive, especially in markets where the cost of living is high, such as in Manhattan, N.Y. Because of the high labour costs and low productivity levels in such environments, it is also desirable to employ as much factory pre-fabrication as possible. This improves quality, increases productivity of site laborers and reduces damage and loss of materials due to weather conditions, dirt, and storage and handling activities on the construction site.
There is therefore a need for an improved window wall or curtain wall system, provided with consideration for the problems outlined above.